1846.]' 



SHARPE ON SLATY CLEAVAGE. 



83 



sides being drawn out and the other contracted till it takes the form 

 c a d of fig. 9. In most cases these two operations appear to have 

 acted uniformly together ; the contraction of the one side being 

 in proportion to the expansion of the other. 



The origin of the oblique pressure on these fossils is easily found: 

 the expansion of the masses of rock in the direction of the dip of the 

 cleavage must cause an oblique pressure on the surface of every bed 

 which is cut obliquely by the cleavage; the fossil shells lying between 

 an expanding mass and a resisting weight of matter have given 

 way before the pressure in the manner described. But as the ex- 

 pansion of the rock in the one direction may have been caused by 

 its compression in the contrary direction, it follows that all the 

 effects yet described may have originated in the compression of the 

 mass of the rock in a direction perpendicular to the cleavage planes. 



The next cases chosen for illustration are those where the angle 

 of incidence of the planes of bedding and cleavage is below 5°. 

 These need not detain us long as they are exactly the same as the 

 last in principle, and only differ in the distortion being more exag- 

 gerated. The specimens are from Tintagel, and belong to Spirifer 

 giganteus of Sowerby. The shells are placed in the same position 

 relatively to the cleavage as before: that of fig. 10 is analogous to 

 that of fig, 6; the wing jpj„^ Iq^ 



DAB lengthened and so 

 much flattened that the ribs - — 

 are nearly lost; the other 

 wing CAB contracted and 

 with the ribs well-marked. 

 The shell fig. 1 1 is more dis- ^ 

 torted ; and as the two valves 

 are united, the change of 

 form can be determined 

 accurately: fig. 12 shows ^ 

 a longitudinal section of '. 

 this shell. The left wings 

 DAB are nearly equally 

 elongated in both valves in 



the direction of the dip of 



the cleavage Y Y, their ex- 

 tent, as seen in the section, 

 being D E and D F ; the 

 iwo right wings are so much 

 contracted that in the sec- 

 tion they only reach from C to E and F. Many other specimens 

 show a similar result in different degrees ; but the most extraordinary 

 that I have seen is the S. giganteus^ as represented in fig. 13, from 

 a bed at Tintagel intersected by the cleavage at an angle of about 

 1°. In this cast the elongated half of the hinge-line DA is three 

 times the length of the other half C A : the hinge area and foramen 

 are of an unnatural size, while a great part of the body of the shell 

 is lost. There is here the most distinct proof that a flexible cast of 

 the shell has been pressed forward towards the upper line Z Z. 



g2 



